We have a turbo diesel and turbo petrol version of the same car (mk4 Golf) in our household. There is 100cc difference in capacity, and there is 20 bhp difference in power, in favour of the petrol engine.
The turbo petrol is a very flexible unit which delivers 220NM of torque at 2000 RPM. It has a 20 bhp advantage over the diesel, and a redline set 2000 RPM higher. This means it feels lively and punchy at day to day revs, but it can't match the diesel which makes 310NM of torque at a similar 1900 RPM peak. Even if we had the higher tuned (180 bhp) petrol engine, it would still fall short of the diesel's torque figure by 70 NM.
Only when you get above 4000 RPM and the diesel has had enough, does the petrol pull out any advantage at all, as it keeps making more and more power right up to its 5700 RPM peak, plus a still useful amount a few hundred RPM later too. The diesel has really had enough by 4000 RPM, and the final 500 to the redline requires a complete lack of mechanical sympathy.
The petrol is faster outright, no question about it. But the diesel feels far quicker at day to day revs, simply because it is. Nearly a third more twisting effort to play with.
The diesel's big limitation is its soft throttle response (compared to the petrol) and its limited rev range. The petrol's big limitation is that you need to use almost all the revs to get the best out of it. So which is best depends on which limitation applies in your daily driving.
Apart from noise and vibration, one area where the petrol does win is in its response. I have always thought you can feel the difference in output control methods between petrol and diesel. With a petrol engine you are throttling its air supply, and to rev / accelerate, you are effectively removing the restriction which always makes it feel keener than the diesel. That's probably psychological on my part though.
Oh, and I've just filled the diesel up in the last half hour, and despite heinous traffic and a mad blast home two days ago using all the power and revs, it's averaged a genuine 47 mpg. The petrol in similar circumstances would be lucky to hit 30.
Last edited by: DP on Fri 21 Jan 11 at 13:26
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